1. The Field of the Invention
The present invention relates to inlet valves used in internal combustion engines. More particularly, the present invention relates to inlet valves coated with a high temperature coating and internal combustion engines incorporating same.
2. Related Technology
Internal combustion engines are used in many different applications, such as automobiles, ships, electric generators, pumps, among others. Inlet valves are a common component of many internal combustion engines. The inlet valves are positioned in an inlet port to close the air intake during combustion. During the air intake stroke, a cam pushes the inlet valve open and allows the fuel mixture to enter the combustion chamber.
The seal that the inlet valve makes with the inlet port is important to engine performance and efficiency. If the valve leaks the pressure in the combustion chamber decreases and the engine generates considerably less power. Engine manufacturers over the last few decades have dedicated substantial efforts in designing inlet valves that can form a tight seal with the valve seat of the inlet port.
FIG. 1 shows a typical inlet valve 10. The body of inlet valve 10 includes a valve head 12 and a valve stem 14. The valve head 10 has a valve seat 16 that is shaped and configured to engage an inlet port to seal a combustion chamber. The valve seat is particularly crucial for the reliability of the inlet valve. It is well-known that corrosion of the seat face can cause the valve to leak when the valve is closed, which results in “burn through.”
To prevent burn through, the valve seats on the inlet valve and the inlet port have been made with increasingly harder materials. The valve seat is made harder by applying a hard cladding layer on the valve head and machining to make the valve seat. The hard cladding makes the valve seat 16 more wear-resistant and reduces the formation of dent marks. Examples of materials that are frequently used for valve seat materials are metal alloys having cobalt and nickel (e.g., Stellite).
The use of extremely hard materials for valve seats has dramatically improved the performance and durability of inlet valves. However, in almost all cases, the advantages of using these hard materials throughout the valve (e.g., in the valve stem and other parts of the valve head) are not sufficient to offset the increase in price over softer metals such as low carbon steel. Consequently the valve stem 14 and the bell region 18 between the seat face 20 and the valve stem 14 are typically made from low carbon steel.
Improvements in the durability of the seat face of inlet valves has allowed inlet valves to hold up under more severe conditions and for longer periods of time than previous inlet valves. In many cases, the seat face is no longer the location where the valve fails. One point of failure observed in inlet valves occurs at the interface where the hard seat material ends and the softer steel of the valve body begins. Gas impingement and/or corrosion can pit or otherwise wear down the material in the valve body over extended periods of time. Wear is particularly likely to happen at the interface because of the transition between the two types of metals and the superior quality of the valve seat material.
Failure at the interface between the harder seat material and the softer valve body material is particularly problematic in engines that use exhaust gas recirculation (EGR). EGR is used to recycle gases that have high quantities of unburned particulate, NOx, and/or SOx back into the combustion chamber to be reburned. The hot and corrosive exhaust gases are mixed with outside air and enter the combustion chamber by passing around the valve head of the inlet valve. Consequently, the recycled exhaust gases come into contact with the bell region 18 of the inlet valve 10. Furthermore, condensation during shutdown can cause pooling of corrosive liquids such as sulfuric acid on the valve. Corrosion and/or gas impingement can weaken bell region 18, and in particular the valve body material at the interface with the hard seating material. Even where a portion of the bell is coated as in U.S. Pat. No. 5,662,078, corrosive gases in contact with the valve body material at the seat-material, body-material interface can cause pits and cracks to form that lead to valve failure at the interface.